This invention relates to reflector type radio frequency (RF) antennas used for communications purposes and more particularly to such antennas having a parabolic shaped reflector, a shroud and a radome.
Parabolic reflector type antennas developed and currently produced for satellite ground applications use are made by forming aluminum sheet on a mandrel by a spinning process. Satellite earth terminal antennas may be forty feet or more in diameter and are fabricated using preformed metal panels or are plastic molded on parabolic forms. The plastic reflectors are metalized to make the surface reflective using any of many well known techniques.
High tolerance parabolic reflectors for high efficiency operation must be very stiff to maintain the manufactured parabolic shape regardless of single or multi-piece construction or size. Reflectors of two or more piece construction of metal or plastic require greater effort and costs than the same size reflector of single piece construction for the same surface precision. However, two or more piece construction of reflectors of diameter larger than about two feet have significant saving in storage and shipping. All must meet the same operating surface tolerances.
One class of parabolic reflector antenna that is known to produce superior radiation performance is the so called "high performance" antenna. This class consists of a parabolic reflector, a reflective cylindrical shroud and a planar radome covering the shroud aperture. Cylindrical shrouds are a development of "tunneling" techniques to reduce unwanted side and back radiation from a transmitting antenna and unwanted side and back radiation reception by a receiving antenna. The shroud as used on high performance parabolic reflector antennas has been a cylinder the same diameter as the reflector to which it attaches at one end and the radome covers the other end of the cylinder.
The radome is a transparent microwave window and may be rigid plastic or a pliable material mated to the cylindrical shroud open aperture by various means.
High performance signifies much reduced side and rear radiation characteristics of the shrouded antenna, as compared with a standard, (average), unshrouded parabolic reflector antenna. Low side and rear radiation performance is required in urban or congested areas to minimize interference with other antennas.
Shrouds of other shapes, such as conical shapes are not as effective as cylindrical shrouds whose reflective surface is parallel to the parabolic reflector axis of the antenna system. Conical shaped shrouds serve as extensions of the parabolic reflector surface and exhibits similar diffraction radiation as simple parabolic apertures.
A shrouded parabolic reflector antenna is relatively more expensive to make than one without a shroud. Typically the shroud is supported by the reflector and adds significant weight and wind force loading to the antenna support, which tends to distort the shape of the parabolic reflector and the reflective shroud, losing some of the surface accuracy produced at the factory. With a cylindrical shroud, radiation to the side and rear can be 20 db or more down as compared to an unshrouded antenna, all else being equal. Thus, the cylindrical shroud does not contribute to structural integrity and does not enhance the parabolic reflector surface accuracy. It is employed when much improved radiation directivity is desired over the performance of an unshrouded parabolic reflector antenna.
For sizes larger than about two feet in diameter, high performance antennas suffer the disadvantage of: greater weight, larger shipping and storage volumes, assembly and installation complexities, and greater cost. As a consequence, high performance antennas have been used only for terrestrial point-to-point microwave communication links.